For mobile data communications, layered communication networks may be deployed as e.g. described by the 3GPP specification (3rd Generation Partnership Project). The 3GPP specification comprises three distinct layers: an application layer, a network control layer and a connectivity layer. The application layer supports end-user applications and may be implemented in mobile stations or application servers in the network. The application layer interfaces with the network layer to enable designing and implementing different services and applications. The network control layer supports communicating services across e.g. different types of networks such as circuit-switched domain networks based on the GSM standard using e.g. an ISDN-related technology or packet-switched networks employing e.g. the GPRS (General Packet Radio Service) technology. The connectivity layer is a transport layer capable of transporting any type of service via e.g. voice, data and multimedia streams.
The 3GPP specification defines the Long Term Evolution (LTE) as an access network technology. The LTE comprises a MME (Mobility Management Entity) forming a control-entity suitable for signalling and selecting a serving gateway (SGW) which routes and forwards data packets carrying e.g. multimedia services. For delivering the multimedia services across the access network, the 3GPP specification defines the IP Multimedia Subsystem (IMS). The IMS comprises a service layer forming an application layer, a control and connectivity layer forming a network control layer, and an access layer forming a connectivity layer. The control and connectivity layer comprises call session control functions (CSCF) forming central entities for Session Initiation Protocol (SIP) signalling. According to the IMS, an application server is provided for hosting and executing services for user entities. An example of an application server is the Multimedia Telephony Service server (MMTel) offering multimedia communications such as voice. Another example of an application server is the Voice Call Continuity Server (VCC).
According to the IMS, the application server interfaces with the Serving CSCF (S-CSCF) using the Session Initiation Protocol (SIP). The S-CSCF transmits a SIP invite message towards the application server in order to establish a communication link towards a user entity. The SIP invite message addresses the user entity using a public identification such as Mobile Subscriber Integrated Services Digital Network Number (MSISDN). If the user entity is available, then the application server returns the SIP invite message towards the S-CSCF. The S-CSCF forwards the received SIP invite message towards the user entity.
Another access technology is Wireless Local Area Network (WLAN). When a user entity, e.g. a mobile phone or a smart phone, enters a transmission range of a WLAN Access Point (WLAN AP) of the WLAN, then the access point may check a registration of the user entity with the WLAN, such as exemplarily described in the EP 1 523 208 A1. If the user entity is registered with the WLAN, then the user entity may offload at least a part of its traffic to the WLAN to be connected to other communication networks, like the internet. Thus, the communication link between the user entity and its mobile communication network may be unloaded. However, the user entity needs to register with the WLAN prior to traffic offloading.